Add Difficulty Gradient Map Editor

[sbodegraven]

I've created a basic web app that lets you paint colours on the Morrowind map to set difficulty levels, which can be exported to an image or JSON. See README.md for details on how to use it.

Related issue: #51

This could definitely be improved on but it should hopefully do the job. Let me know if I should add/change anything.

The exported JSON looks like this (except much longer), where d is the difficulty between 0 and 1:

[
  {
    "x": -28,
    "y": 27,
    "d": 0.6
  },
  {
    "x": -28,
    "y": 26,
    "d": 0.94
  }
]

[sbodegraven]

Known issue: brush shape outlines may be visible when drawing. This happens because the brush erases from the drawing layer before drawing again so that the colours don't compound and cause it to lose transparency. I have not had any luck finding a solution for this, though this should not have any real impact on the difficulty.